Neodymium doped, end pumped, substantially pure fused silica glass (hard glass) fiber lasers and/or amplifiers have been considered previously in the art. However, due to the inability of those skilled in the art to provide appropriately high concentrations of neodymium doped hard glass, an optimal design for this type of laser and/or amplifier has not been found.
For practical reasons, it is desirable that the length of the glass fiber for such lasers and/or amplifiers not be excessively long nor too short to handle. In particular, it is desirable to fabricate a laser and/or amplifier having a relatively short fiber and having a high enough concentration of neodymium so that substantially all the incident pump light focused on an end is absorbed in one or, at most, two passes through the fiber. More specifically, if the apparatus is used as a laser, substantially all of the incident pump radiation should be absorbed in one or two passes through the fiber, whereas if the apparatus is used as an amplifier, substantially all of the incident pump radiation should be absorbed in a single pass through the fiber.
However, there is a practical upper bound on the neodymium concentration which is dictated by factors other than difficulty of fabrication. In practice, the neodymium concentration is limited because a high concentration leads to concentration quenching, concentration quenching being the physical phenomenon where non-radiative decays occur from an excited state. In neodymium doped glass fibers, concentration quenching occurs as a result of the interaction of active neodymium ions with each other.
Concentration quenching is undesirable because it, in effect, decreases the amount of pump power available to drive the laser and/or amplifier. This results in a decrease in quantum efficiency for fluorescence and directly relates to a decrease in the performance of the laser and/or amplifier. In other words, concentration quenching acts as if the pump power that was available from the pump source has been decreased by an amount which is equivalent to the decrease in the fluorescence efficiency of the laser and/or amplifier.
While the effective decrease in fluorescence efficiency caused by concentration quenching is seen to cause an effective decrease in the power efficiency of the pump source, it is important for another reason. This reason is that the power efficiency of the pump source determines how much input power is required of the pump and, therefore, determines certain heat dissipation requirements. Poor efficiency means higher input power requirements for a given optical power output. In general, this can present power supply problems for remotely located equipment. Further, inefficiently converted pump energy results in heating which requires appropriate heat dissipation arrangements which would otherwise be unnecessary.
In light of the above, a need exists for an efficient, high concentration, neodymium ion doped, end pumped, substantially pure fused silica glass fiber laser and/or amplifier.